Martian elusive Pits and the challenge of working remotely

Post by Dr. Andreas Johnsson, University of Gothenburg, Sweden

Geomorphologists working with Mars share a frustration of not being able to visit their objects of investigation. To counter this, a commonly used approach is to look for environments on Earth that resemble those studied on Mars. This approach, called Earth-analogue studies, helps to guide our line of reasoning in deciphering formation mechanisms of specific martian landforms of interest. Mars, being the most earth-like planet in the solar system hosts numerous landscapes and landforms that in plan-view show remarkable similarities to known features on Earth. Especially striking examples are martian glacial flow-like features and gullies to that resemble terrestrial glaciers and fluvially-incised ravines, respectively. As a consequence their Earth counterparts have been studied with great intensity for the last couple of decades. Although correspondences in form may guide our way of thinking of plausible formative processes by reference to Earth, the approach is not without pitfalls. For example, experimental studies in Mars climate chambers have shown that fluvially triggered slope processes may be of a completely different nature under Mars’ atmospheric conditions of low pressure combined with low temperatures, but the resulting landform looks about the same. This is a problem of equifinality (i.e. convergence of form), something that also terrestrial scientists encounter but which is a major challenge in planetary geomorphology (e.g. Hauber et al. 2011; Zimbelman 2001). One way to try to minimize equifinality is by taking whole landform assemblages into account where different types of landforms may have some genetic linkages.

Although Mars poses challenges like these to planetary geomorphologists and alike on a daily basis it also hosts landscape features that lack any obvious terrestrial morphological counterparts (see the Image of the Month by Berhardt for another example). This makes the endeavor of understanding the evolution of the martian surface even more challenging but also very exciting. One such feature is simply called a pit (Orgel et al. 2019) (Image 1).

Pit collage

Image 1: Different morphological varieties of pits located in Acidalia Planitia. A) A pair of almost circular pits. Rims show a high abundance of large boulders. Note the outline of multiple ridges in the lower part of pits (HiRISE ID: ESP_019783_2115_RED). B) Several elongated irregular pits that show no obvious overlapping of rim sediments (HiRISE ID: ESP_026521_2130_RED). C) A cluster of several pits contracted on the equatorward slope of a topographic feature. The center pits has two sets of rims (HiRISE ID: .ESP_026521_2130_RED). Image credits: NASA/JPL/UofA for HiRISE.    

Overview

Image 2: Overview map of the great northern plains of Acidalia Planitia. The white circle shows the approximate locations of most documented pits. Credit: Google Mars.

Pits are enigmatic features that occur mostly in the southern part of Acidalia Planitia on Mars, to the west and northwest of Cydonia Mensae (Image 2). Pits have circular, elongated, or irregular plan view shapes with raised rims and they reach diameters of up to a few hundred meters. They are preferentially located on equator‐facing slopes such as on the inner walls of impact craters or on any other landform with positive topography. They are found on regional slopes up to 3.2°, but more often on gentler slopes between 0.7° and 1.8°. They generally form clusters or chains but isolated features exist as well. The lack of superposition of the pit ridges suggests that many individual pits in a pit chain seem to have formed more or less simultaneously (Image 3). On the other hand, smaller pits may occasionally be nested in larger ones, suggesting that multiple episodes of pit formation may have occurred. The pits appear to be relatively shallow and the raised rims partly consist of boulder‐sized blocks. The floors of some pits are partly covered by aeolian bedforms and there is no evidence of fluvial activity or runoff.

Pits are concentrated between 24°N and 36°N and their occurrence does not overlap with the latitude-dependent mantle, an atmospherically derived deposit of ice and dust that blankets large swaths of the mid-latitudes.

Pits_chain2

Image 3: A chain of pits located on the equator-facing slope of a local ridge (HiRISE ID: PSP_PSP_008641_2105_RED). The pits appear young based on the well-preserved rims. Some pits show limited eolian deposition and ripple forms. Image credit: NASA/JPL/UofA for HiRISE.

The origin of these elusive landforms is not fully understood. Based on their physical setting, orientation and careful examination of their morphology a few mechanisms may explain their origin, while other mechanisms can be excluded. An early idea is the formation due to collapsing ground, since they are sometimes associated with graben systems (Davatzes, A., 2008. https://www.uahirise.org/PSP_008641_2105). However, the location of pits in craters and on other topographic features seems to contradict this.  Prior to the advent of HiRISE imagery, it was proposed that the pits where formed in a similar manner as terrestrial aeolian blowouts (Kuznetsov et al., 2005). However, at the time satellite images could not resolve the boulder-sized components of the rim material, which is inconsistent with sediments being mobilized by the wind. Also, the irregular plan view shape of many the pits would argue against a formation as secondary craters or volcanic features.

Instead, the preferred equatorward orientation may suggest a control by insolation. Moreover, the raised rims may argue for a high-energy excavation by some unknown agent. One possibility is gas jets produced by sublimation of CO2 under a slab of dry ice. However, thick slabs of dry ice are not expected at the low latitudes where pits occur. Another possibility is that they represent remnant landforms from when the dust/ice rich mantling unit extended further south, or insolation controlled water release (Orgel et al., 2019). However, their true origin still remains elusive.

Further Reading

Zimbelman, J.R., 2001. Image resolution and evaluation of genetic hypotheses for planetary landscapes. Geomorphology 37, pp. 179–199.

Hauber, E. et al., 2011. Periglacial landscapes on Svalbard: Terrestrial analogs for cold-climate landforms on Mars. In: Garry, W.B., Bleacher, J.E. (Eds.), Analogs for Planetary Exploration. Geological Society of American Special Publication 483, pp. 177–201.

Orgel, C. et al., 2019. Gridmapping the Northern Plains of Mars: A New Overview of Recent Water‐ and Ice‐Related Landforms in Acidalia Planitia. https://doi.org/10.1029/2018JE005664

Kuznetsov, I.V., Kuzmin, R.O., and Greeley, R., 2005. Wind-related erosion depressions within a small impact craters in Chryse and Elysium Planitiae on Mars. 36th Lunar and Planetary  Science Conference, Houston, TX, #1810.

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